As a method of manufacturing seamless steel tubes under hot working conditions, the Mannesmann tube-making process is widely employed. In this tube-making process, a round billet heated to a high temperature is fed as a material to be rolled into a piercing mill (a so-called "piercer"), which pierces the axial center portion of the round billet to obtain a hollow shell. The thus-obtained hollow shell is fed, directly or as needed after undergoing an expansion or wall-thinning process in an elongator having the same structure as that of the piercing mill, into a subsequent elongating mill such as a plug mill, a mandrel mill, or the like so as to be elongated. Subsequently, the thus-elongated tube undergoes a finishing process provided by a stretch reducer for shape correction, a reeler for polishing, and a sizer for sizing, thereby becoming a seamless steel tube product.
FIG. 1 is a schematic representation of the material to be rolled when it is pierced by the piercing mill. Piercing rollers 1, 1 are axisymmetrically arranged at a predetermined cross angle and feed angle with respect to a pass line X--X along which is rolled a round billet 3 serving as a material to be rolled. In the piercing mill having the piercing rollers 1, 1 arranged in the previously-described layout, the round billet 3 fed along the pass line X--X in the direction indicated by an outlined arrow is brought into gap with the piercing rollers 1, 1 and travels over the pass line while in rotation. A hole is pierced through the axial center of the of the billet 3 by a plug 2, thereby forming a hollow shell. During the piercing process, the plug 2 is supported by a mandrel supporting apparatus (not shown) so as to be positioned between the piercing rollers 1, 1 along the pass line.
As described above, the round billet 3 serving as the material to be rolled is intensively rotated when it is pierced. For this reason, if there is a bend in the round billet 3, or if the round billet 3 is bent as a result of having been unevenly cooled after a heating process, large deflections will develop in the round billet 3 when it undergoes the piercing and rolling processes. If large deflections develop in the round billet 3, the round billet 3 vigorously hits an entrance conveyor section of the piercing mill, and violent vibrations and loud noise arise. This may render the piercing and rolling operations unstable and generate flaws in the outer surface of the pierced hollow shell.
In general, in order to prevent such an accident, a cannon 4 serving as a cylindrical guide is disposed at the entrance of the piercing mill along the pass line X--X, as illustrated in FIG. 1. Even if deflections are caused by a bend in the round billet 3, the round billet 3 is rotated within the cannon 4, thereby ultimately protecting the outer surface of the hollow shell.
Since the cannon 4 disposed at the entrance of the piercing mill absorbs deflections in the round billet 3, it must be fixedly mounted on the main frame of the piercing mill. In contrast, in order to absorb the deflections in the round billet 3, it is necessary to maintain a constant clearance between the inner diameter of the cannon 4 and the round billet 3. For these reasons, it is necessary to exchange the cannon 4 every time the size of the round billet 3 serving as the material to be rolled is changed. Further, the round billet 3 is maintained at a high temperature when undergoing the piercing and rolling processes, and the inner surface of the cannon 4 is considerably damaged by the deflections in the round billet 3. Therefore, even if the size of the round billet 3 still remains unchanged, it is necessary to periodically exchange the cannon 4 in view of maintenance.
The cannon 4 of the conventional piercing mill is manually exchanged by use of a crane or a jib crane disposed at the entrance of the piercing mill. The manual exchange of the cannon 4 requires a large number of steps, thereby inevitably resulting in a reduction in the availability of the piercing mill. Particularly, in the case of recent continuous Mannesmann tube manufacturing facilities aimed at highly efficient production of seamless steel tubes, exchange of a cannon results in a reduction in the overall efficiency of manufacture of seamless steel tubes.